The invention relates to new liquid crystalline compounds, mixtures of those compounds and their application in optical devices. More particularly, it relates to the use of a component of a polymerisable liquid crystalline mixture in the production of orientated liquid crystalline polymers; compounds used as components in polymerisable liquid crystalline mixtures; liquid crystalline mixtures comprising these components, liquid crystalline polymers prepared from such components; and liquid crystalline devices comprising those compounds.
Liquid crystal polymers (LCPs) are used in the manufacture of optical components such as waveguides, optical gratings, filters, retarders, piezoelectric cells and non-linear optical cells and films. The choice of LCP for use in any one of the aforementioned optical components depends upon its associated optical properties such as the optical anisotropy, refractive index, transparency and dispersion. Optical filters, for example, contain LCPs having a large anisotropy (xcex94n) and a low dispersion (n=f(xcex)).
In some applications there is a requirement to produce LCPs in which the component molecules adopt a specific tilt angle or orientation with respect to the plane of the substrate or to a plane perpendicular to the substrate. These LCP materials can be used as optical components such as compensation layers and retarders. Such optical components may be used in the production of liquid crystal devices (LCDs) with improved viewing angles, for example.
LCPs are manufactured by orientating a layer of a polymerisable liquid crystal single compound or mixture on an orientated substrate and cross-linking the mesogenic layer to form a liquid crystal polymer (LCP) network. Polymerisable LC compounds used in the manufacture of the LCPs need to be chemically and thermally stable, stable to electromagnetic radiation, soluble in standard solvents and miscible with other LC components, and to exhibit liquid crystalline properties over the range 25 to 150xc2x0 C., preferably 25 to 80xc2x0 C. The configuration imposed by the orientation layer on the polymerisable LC single compound or mixture becomes fixed or frozen into the LCP network formed upon cross-linking. The resulting LCP films have a high viscosity and are stable to mechanical stresses, temperature and light exposure.
There is therefore a need for a liquid crystalline single compound or mixture which exhibits a broad liquid-crystalline thermal range and which can be orientated on a substrate prior to cross-linking in such a way that the orientation of the LC single compound or mixture on the substrate remains stable over the period required for manufacturing the LCP network. Components which may be used in photo-crosslinkable liquid crystalline layers are particularly desirable.
Compounds known from the prior art include those disclosed in EP-A-0675186, EP-A-0700981 and EP-A-0748852 (all F. Hoffmann-La Roche A G). The three earlier documents disclose compounds such as (taken from EP-A-0675186): 
U.S. Pat. No. 3,971,824 (Van Meter et al./Eastman Kodak Company) discloses in its broadest aspect compounds of the following formula: 
though in fact there is no enabling disclosure of anything other than the following: 
where X is chlorine.
Previous strategies used for obtaining the desired thermal and optical properties with a given LCP material have mainly relied upon mixtures of compounds comprising at least one liquid crystalline polymerisable monomer and the combination of their individual properties. However due to the general incompatibility of the latter components at the molecular scale, the thermodynamic behaviour of their corresponding mixtures is generally undesirable (for example, a depression of the clearing point, a reduction of the liquid crystalline range etc.), besides some problems of miscibility between the different components of the mixture leading to difficulties in achieving a uniform orientation of the LCP material. To ameliorate this situation, a new concept of obtaining LCP materials of special thermal and optical properties was investigated. This concept uses chemical junctions at lateral positions of different molecules, at least one of them being mesogenic, having each one or more of the properties which are required in the final LCP material. Depending on the application, these properties can be selectively induced from at least one of the mesogenic stairs of the new xe2x80x9cstaircase moleculesxe2x80x9d.
Thus the invention provides chiral or achiral xe2x80x9cstaircasexe2x80x9d compounds of formula I: 
wherein:
A1 to A6 each independently represent hydrogen; an optionally-substituted methyl group; or an optionally-substituted hydrocarbon group of 2 to 80 C-atoms, in which one or more C-atoms may be replaced by a heteroatom, in such a way that oxygen atoms are not linked to one another;
B1 and B2 each independently represent a single bond, an oxygen atom or an optionally-substituted hydrocarbon group of 1 to 80 C-atoms, in which one or more C-atoms may be replaced by a heteroatom, in such a way that oxygen atoms are not linked to one another;
MG1 and MG3 each independently represent an optionally-substituted aliphatic group with 1 to 80 C-atoms, in which one or more C-atoms may be replaced by a heteroatom, in such a way that oxygen atoms are not linked to one another; or an optionally-substituted aromatic or non-aromatic carbocyclic or heterocyclic ring system; with 1 to 80 C-atoms;
MG2 represents a group comprising at least two and up to four optionally-substituted aromatic or non-aromatic carbocyclic or heterocyclic ring systems, with 1 to 80 C-atoms, wherein, when MG2 represents a group comprising four optionally-substituted ring systems, at least three of the ring systems are aligned in between B1 and B2;
n1 and n2 are each independently 1 or 2, where xe2x80x9cn1=2xe2x80x9d (or xe2x80x9cn2=2xe2x80x9d) indicates the presence of two separate linkages via the groups B1 (or the groups B2) between the groups MG1 and MG2 (or MG2 and MG3); and
n3 is a positive integer up to 1000;
with the proviso that:
when A3 and A4 both represent hydrogen, then both MG1 and MG3 represent an araliphatic group with 1 to 80 C-atoms, in which one or more C-atoms may be replaced by a heteroatom, or an optionally-substituted aromatic or non-aromatic carbocyclic or heterocyclic ring system, with 1 to 80 C-atoms; and at least two of A1, A2, A5 and A6 each independently represent an optionally-substituted hydrocarbon group of 3 to 80 C-atoms, in which one or more C-atoms may be replaced by a heteroatom;
when A1, A2, A5 and A6 all represent hydrogen, then A3 and A4 both represent an optionally-substituted hydrocarbon group of 3 to 80 C-atoms, in which one or more C-atoms may be replaced by a heteroatom; and
when MG2 represents a group comprising two or three optionally-substituted ring systems, then neither of A3 and A4 includes an aromatic ring.
The term xe2x80x9caliphaticxe2x80x9d includes straight-chain and branched alkylene, as well as saturated and unsaturated groups. Possible substituents include alkyl, aryl (thus giving an araliphatic group) and cycloalkyl, as well as amino, cyano, epoxy, halogen, hydroxy, nitro, oxo etc. Possible heteroatoms which may replace carbon atoms include nitrogen, oxygen and sulphur. In the case of nitrogen further substitution is possible with groups such as alkyl, aryl and cycloalkyl. Likewise, the termsxe2x80x9calkylxe2x80x9d and xe2x80x9calkylenexe2x80x9d, as used herein, includes straight-chain and branched groups, as well as saturated and unsaturated groups.
When MG2 represents a group comprising four optionally-substituted ring systems, at least three of the ring systems are aligned in between B1 and B2. Thus, at least three of the rings are all in a discrete identifiable block positioned in between B1 and B2, not in an arbitrarily defined region with some of the rings protruding from the axis of the molecule connecting B1 with B2. Comparison may be made with compounds such as Compound I-a of EP-A-0675186: 
which could be drawn with an arbitrarily defined Z-shaped central portion having four rings. Two of these rings protrude from the axis of the molecule connecting the remaining arms of the molecule, and the compound falls outside the scope of the present invention.
When MG2 represents a group comprising two or three optionally-substituted ring systems, then neither of A3 and A4 includes an aromatic ring. This again makes it clear that with compounds such as Compound I-a of U.S. Pat. No. 5,567,349 fall outside the scope of the present invention. It is not possible to produce the present invention by arbitrarily labelling parts of molecules known from the prior art.
The groups MG1, MG2 and MG3 in the new xe2x80x9cstaircase moleculesxe2x80x9d may be selected so that each has one or more of the properties which are required in the final LCP material. To an expert in liquid crystals, the molecular architecture of compounds of formula I would not have been thought to be favourable for obtaining a liquid crystalline mesophase, because a bulky substituent at a position lateral to a mesogenic core would have been thought to cause a loss of mesogenic character. However we have now discovered that the compounds of formula I were surprisingly found to be liquid crystalline over a broad thermal range. Besides, they are suitable for producing a high tilt and high optical birefringence together with well oriented LCP films.
MG2 may be either a mesogenic group or otherwise.
It is advantageous for the liquid crystalline compounds to be photo-crosslinkable, so that for example they may be used in a crosslinked state in optical devices. Thus preferably at least one of A1 to A6 includes a polymerisable group. In such a case, when A3 and A4 both represent hydrogen then at least one of A1, A2, A5 and A6 would include a polymerisable group; whereas when A1, A2, A5 and A6 all represent hydrogen then at least one of A3 and A4 would include a polymerisable group.
In a first preferred embodiment of the present invention, each or any of the groups A1 to A6 may be selected from a residue of formula (II):
Pxe2x80x94(Sp1)k1xe2x80x94(X1)t1xe2x80x94xe2x80x83xe2x80x83(II)
wherein:
P is hydrogen or a polymerisable group selected from groups comprising CH2xe2x95x90CWxe2x80x94, CH2xe2x95x90Wxe2x80x94Oxe2x80x94, CH2xe2x95x90CWxe2x80x94COOxe2x80x94, CH2xe2x95x90C(Ph)-COOxe2x80x94, CH2xe2x95x90CHxe2x80x94COO-Ph-, CH2xe2x95x90CWxe2x80x94COxe2x80x94NHxe2x80x94, CH2xe2x95x90C(Ph)-CONHxe2x80x94, CH2xe2x95x90C(COORxe2x80x2)xe2x80x94CH2xe2x80x94COOxe2x80x94, CH2xe2x95x90CHxe2x80x94Oxe2x80x94, CH2xe2x95x90CHxe2x80x94OOCxe2x80x94, (Ph)-CHxe2x95x90CHxe2x80x94, CH3-Cxe2x95x90Nxe2x80x94(CH2)m3xe2x80x94, HOxe2x80x94, HSxe2x80x94, HOxe2x80x94(CH2)m3xe2x80x94, HSxe2x80x94(CH2)m3xe2x80x94, HO(CH2)m3COOxe2x80x94, HS(CH2)m3COOxe2x80x94, HWNxe2x80x94, HOC(O)xe2x80x94, CH2xe2x95x90CH-Ph-(O)m4 
wherein:
W represents H, F, Cl, Br or I or a C1-5 alkyl group;
m3 is an integer having a value of from 1 to 9;
m4 is an integer having a value of 0 or 1,
Rxe2x80x2 represents a C1-5 alkyl group; and
Rxe2x80x3 represents a C1-5 alkyl group, methoxy, cyano, F, Cl, Br or I;
Sp1 represents an optionally-substituted C1-20 alkylene group, in which one or more C-atoms may be replaced by a heteroatom;
k1 is an integer having a value of from 0 to 4;
X1 represents xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94NHxe2x80x94, N(CH3)xe2x80x94, xe2x80x94CH(OH)xe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94CH2(CO)xe2x80x94, xe2x80x94SOxe2x80x94, xe2x80x94CH2(SO)xe2x80x94, xe2x80x94SO2xe2x80x94, xe2x80x94CH2(SO2)xe2x80x94, xe2x80x94COOxe2x80x94, xe2x80x94OCOxe2x80x94, xe2x80x94OCOxe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94COxe2x80x94, xe2x80x94COxe2x80x94Sxe2x80x94, xe2x80x94SOOxe2x80x94, xe2x80x94OSOxe2x80x94, xe2x80x94SOSxe2x80x94, xe2x80x94CH2xe2x80x94CH2xe2x80x94, xe2x80x94OCH2xe2x80x94, xe2x80x94CH2Oxe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94, or xe2x80x94Cxe2x89xa1Cxe2x80x94; and
t1 is an integer having a value of 0 or 1;
with the proviso that at least one of the groups A1 to A6 is not a hydrogen atom.
In relation to the residue of formula (II), the term Ph is to be understood as denoting phenylene and (Ph) as denoting phenyl.
The C1-20 alkylene group Sp1 may comprise branched or straight chain alkylene groups and may be unsubstituted, mono- or polysubstituted by F, Cl, Br, I or CN. Alternatively or in addition one or more of CH2 groups present in the hydrocarbon chain may be replaced, independently, by one or more groups selected from xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94NHxe2x80x94, N(CH3)xe2x80x94, xe2x80x94CH(OH)xe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94CH2(CO)xe2x80x94, xe2x80x94SOxe2x80x94, xe2x80x94CH2(SO)xe2x80x94, xe2x80x94SO2xe2x80x94, xe2x80x94CH2(SO2)xe2x80x94, xe2x80x94COOxe2x80x94, xe2x80x94OCOxe2x80x94, xe2x80x94OCOxe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94COxe2x80x94, xe2x80x94COxe2x80x94Sxe2x80x94, xe2x80x94SOOxe2x80x94, xe2x80x94OSOxe2x80x94, xe2x80x94SOSxe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94, xe2x80x94(CF2)xe2x80x94r, xe2x80x94(CD2)sxe2x80x94 or C(W1)xe2x95x90C(W2)xe2x80x94, with the proviso that no two oxygen atoms are directly linked to each other. W1 and W2 each represent, independently, H, Hxe2x80x94(CH2)q1xe2x80x94 or Cl. The integers r, s and q1 each independently represent a number of between 1 and 15.
More preferably, A1 to A2 each independently represent a group of formula (III):
P2xe2x80x94Sp5xe2x80x94X4xe2x80x94xe2x80x83xe2x80x83(III);
wherein:
X4 represents xe2x80x94Oxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94COOxe2x80x94, xe2x80x94OCOxe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94, or a single bond, especially xe2x80x94Oxe2x80x94, xe2x80x94COOxe2x80x94, xe2x80x94OCOxe2x80x94 or single bond;
Sp5 represents a C1-20 straight-chain alkylene group, especially ethylene, propylene, butylene, pentylene, hexylene, heptylene, octylene, nonylene, decylene, undecylene, or dodecylene; and
P2 represents hydrogen, CH2xe2x95x90CW5xe2x80x94 or CH2xe2x95x90CW5xe2x80x94(CO)v2Oxe2x80x94,
xe2x80x83wherein:
W5 represents H, CH3, F, Cl, Br or I; and
v2 is 0or 1.
One or more of A1 to A6 may also represent a C1-C20-alkyl, C1-C20-alkoxy, C1-C20-alkoxycarbonyl, C1-C20-alkylcarbonyl or C1-C20-alkylcarbonyloxy group, for example methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl, dodecyl, methoxy, ethoxy, n-propoxy, isopropoxy, butoxy, pentyloxy, hexyloxy, heptyloxy, octyloxy, nonyloxy, decyloxy, undecyloxy, dodecyloxy, methoxycarbonyl, ethoxycarbonyl, propoxycarbonyl, butoxycarbonyl, pentyloxycarbonyl, hexyloxycarbonyl, octyloxycarbonyl, nonyloxycarbonyl, decyloxycarbonyl, undecyloxycarbonyl, dodecyloxycarbonyl, acetyl, propionyl, butyryl, valeryl, hexanoyl, heptanoyl, octanoyl, nonanoyl, decanoyl, undecanoyl, dodecanoyl, terdecanoyl, acetoxy, propionyloxy, butyryloxy, valeryloxy, hexanoyloxy, heptanoyloxy, octanoyloxy, nonanoyloxy, decanoyloxy, undecanoyloxy, dodecanoyloxy, terdecanoyloxy and the like.
In a second preferred embodiment of the present invention each or either of the groups B1 and/or B2 comprises a group of formula (IV):
(X2)t2xe2x80x94(Sp2)k2xe2x80x94(X3)t3xe2x80x83xe2x80x83(IV)
wherein:
Sp2 represents a C1-20 alkylene group;
X2 and X3 each independently represent xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94NHxe2x80x94, N(CH3)xe2x80x94, xe2x80x94CH(OH)xe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94CH2(CO)xe2x80x94, xe2x80x94SOxe2x80x94, xe2x80x94CH2(SO)xe2x80x94, xe2x80x94SO2xe2x80x94, xe2x80x94CH2(SO2)xe2x80x94, xe2x80x94COOxe2x80x94, xe2x80x94OCOxe2x80x94, xe2x80x94OCOxe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94COxe2x80x94, xe2x80x94COxe2x80x94Sxe2x80x94, xe2x80x94SOOxe2x80x94, xe2x80x94OSOxe2x80x94, xe2x80x94SOSxe2x80x94, xe2x80x94CH2xe2x80x94CH2xe2x80x94, xe2x80x94OCH2xe2x80x94, xe2x80x94CH2Oxe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94 or a single bond;
k2 is an integer, having a value of 0 or 1;
t2 and t3 are integers, each independently having a value of 0 or 1;
with the proviso that oxygen atoms are not linked one to another.
Preferably B1 and B2 each independently represent a group of formula (IV), wherein:
X2 to X3 each independently represent xe2x80x94Oxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94COOxe2x80x94, xe2x80x94OCOxe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94, or a single bond, especially xe2x80x94Oxe2x80x94, xe2x80x94COOxe2x80x94, xe2x80x94OCOxe2x80x94 or a single bond; and
Sp2 represents a C1-20 straight-chain alkylene group, especially ethylene, propylene, butylene, pentylene, hexylene, heptylene, octylene, nonylene, decylene, undecylene or dodecylene.
An especially preferred compound is that in which B1 and B2 each independently represent a group of formula (IV) and A1 to A6 each independently represent a group of formula (III).
The invention is particularly useful when MG2 is a mesogenic group and the groups of MG1 and MG3 also impart mesogenic properties to the molecule, in addition to those properties imparted by the mesogenic group MG2. Thus preferably MG2 and at least one of MG1 and MG3 represents a mesogenic group comprising at least two optionally-substituted aromatic or non-aromatic carbocyclic or heterocyclic ring systems.
Preferably MG2 represents a mesogenic group comprising 2 to 4 aromatic or non-aromatic carbocyclic or heterocyclic ring systems and optionally up to 3 bridging groups, and at least one of MG1 and MG3 represent a mesogenic group comprising 1 to 4 aromatic or non-aromatic carbocyclic or heterocyclic ring systems and optionally up to 3 bridging groups. These are more preferably selected from the meanings of formulae V:
C1xe2x80x94(Z1xe2x80x94C2)a1xe2x80x94(Z2xe2x80x94C3)a2xe2x80x94(Z3xe2x80x94C4)a3xe2x80x83xe2x80x83(V),
in which:
C1 to C4 are in each case independently optionally-substituted non-aromatic, aromatic, carbocyclic or heterocyclic groups;
Z1 to Z3 are independently from each other xe2x80x94COOxe2x80x94, xe2x80x94OCOxe2x80x94, xe2x80x94CH2xe2x80x94CH2xe2x80x94, xe2x80x94OCH2xe2x80x94, xe2x80x94CH2Oxe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94Chxe2x89xa1Cxe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94COOxe2x80x94, xe2x80x94OCOxe2x80x94CHxe2x95x90CHxe2x80x94 or a single bond; and
a1, a2 and a3 are independently integers 0 to 3, such that a1+a2+a3xe2x89xa63.
Especially preferred are those in which C1 to C4 are selected from: 
with:
L being xe2x80x94CH3, xe2x80x94COCH3, xe2x80x94NO2, CN, or halogen
u1 being 0, 1, 2, 3, or 4,
u2 being 0, 1, 2, or 3, and
u3 being 0, 1, or 2.
More especially preferred are those in which C1 to C4 are selected from cyclohexylene, phenylene, naphthylene or phenanthrylene.
For ease of synthesis, the molecules may possess some symmetrical aspects. Thus:
A1 and A2 may be identical;
A5 and A6 may be identical;
A1xe2x80x94MG1xe2x80x94A2 and A5xe2x80x94MG3xe2x80x94A6 may be identical;
A3 and A4 may be identical; or
n1, n2 and n3 may equal 1 and B1 and B2 may be identical.
The compounds of the invention may be readily prepared using methods that are well known to the person skilled in the art, such as those documented in Houben-Weyl, Methoden der Organischen Chemie, Thieme-Verlag, Stuttgart. The compounds may for example be made according to the following reaction schemes: 
Based on the synthetic ways drawn in Schemes 1-7, typical examples representing xe2x80x9cstaircasexe2x80x9d derivatives of formula I and shown in the following list of compounds may be prepared. This list is, however, to be understood only as illustrative without limiting the scope of the present invention: 
The xe2x80x9cstaircasexe2x80x9d compounds of formula I disclosed in the foregoing and the following may be used alone or as a component of a liquid crystal mixture. Liquid crystalline LCPs. Another aspect of the invention therefore comprises a liquid crystalline material comprising a compound of formula (I). Preferably the liquid crystalline material comprises at least two components. The additional components must be miscible with the compound of formula (I) and may be selected from known mesogenic materials such as those reported in Adv. Mater. 5, 107 (1993), Mol. Cryst. Liq. Cryst. 307, 111 (1997), J. Mat. Chem. 5, 2047 (1995) or in patent applications U.S. Pat. Nos. 5,593,617; 5,567,349; GB-A-2 297 556; GB-A-2 299 333; DE-A-195 04 224; EP-A-0 606 940; EP-A-0 643 121 and EP-A-0 606 939, optionally selected from EP-A-0 606 940; EP-A-0 643 121 and EP-A-0 606 939.
The form of the liquid crystal material will depend upon the application in which it is to be used and may be present as a liquid crystalline mixture, (co)polymer, elastomer, polymer gel or polymer network. Polymer networks have been found to be of particular use and in a further preferred embodiment of the invention there is provided a polymer network comprising a compound of formula (I). Preferably the polymer network comprises at least two components, at least one of which is a xe2x80x9cstaircasexe2x80x9d compound of formula (I).
The polymer network may be prepared by copolymerisation of a mesogenic mixture comprising:
i) at least one chiral or/and achiral mesogenic polymerisable compound;
ii) at least one xe2x80x9cstaircasexe2x80x9d compound of formula I; and
iii) an initiator.
The chiral or achiral mesogenic polymerisable compound may be a xe2x80x9cstaircasexe2x80x9d compound of formula (I). Alternatively or in addition, the polymerisable compound may be selected from the known mesogenic materials referred to above. Preferably the chiral or achiral polymerisable compound includes the nematic phase in its thermotropic sequence.
The polymer network may optionally comprise further components. These include further polymerisable compounds, stabilisers and dyes. The additional polymerisable compounds preferably comprise a non-mesogenic compound having at least one polymerisable functional group, especially diacrylate compounds.
Any suitable stabiliser that prevents undesired spontaneous polymerisation, for example during storage of the mixture, may be used in the liquid crystalline mixture according to the invention. A broad range of these compounds is commercially available. Typical examples include 4-ethoxyphenol or 2,6-di-tert-butyl-4-methylphenol (BHT).
If colour filters are required, dyes may be added to the mixture. In a preferred embodiment of the invention the LC mixture contains no dye.
The chiral or achiral polymerisable mesogenic compound may be present in an amount comprising 0.01 to 99% by weight of the liquid crystalline polymer network mixture, preferably 50 to 95% by weight.
The xe2x80x9cstaircasexe2x80x9d compound of formula (I) may be present in an amount from 0.1 to 100% by weight of the liquid crystalline network, preferably from 1 to 50% by weight.
The initiator is preferably a photoinitiator and may be a radical or cationic initiator that is present in an amount comprising 0.1 to 5% by weight of the polymer mixture, preferably from 0.2 to 2% by weight.
When the mixture further comprises a stabiliser, this is generally present in an amount comprising 0.01 to 5% by weight of the liquid crystalline mixture, preferably from 0.1 to 1% by weight.
These polymerisable liquid crystalline mixtures may be formed into liquid crystalline polymer (LCP) films and a fifth aspect of the invention provides a LCP film comprising a compound of formula (I). LCP films may be readily prepared by UV polymerisation of a LC mixture according to the invention; a film comprising the LC mixture is formed on a substrate and polymerised using UV light to give a cross-linked liquid crystal polymer (LCP) film. The film is both light and temperature stable and can be used in the manufacture of devices such as waveguides, optical gratings, filters, retarders, piezoelectric cells or thin films exhibiting non-linear optical properties.
Different methods can be used for the formation of the sought LCP network, starting from the polymerisable liquid crystalline mixture manufactured as described above. Transparent substrates such as coated ITO (indium tin oxide), glass or plastic substrates, may be used. Preferred substrates include glass or plastic, especially those including a layer of rubbed polyimide or polyamide or a layer of photo-oriented photopolymer (LPP). Said layers are used to facilitate uniform orientation of the liquid crystalline mixture.
In the preparation of LCP films, it is particularly important to prevent the formation of defects or inhomogenities. This can be achieved by forming the polymerisable liquid crystalline mixture into a thin film; and placing the mixture between two of the aforementioned substrates which are then sheared over a small distance until a planar order was obtained; or capillary filling the polymerisable liquid crystalline mixture between two of the said substrates; prior to curing, for example by UV light, preferably in the presence of a photoinitiator, such as IRGACURE(trademark).
A further aspect of the invention provides an optical or electro-optical component containing a liquid crystalline polymer film comprising a compound of formula (I). The optical or electro-optical component may be a waveguide, an optical gratings, a filter, a retarder, a piezoelectric cell or a non-linear optical cell or film.